Computer and Network Security Rabie A. Ramadan Lecture 2.
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Transcript of Computer and Network Security Rabie A. Ramadan Lecture 2.
Computer and Network Security
Rabie A. Ramadan
Lecture 2
Table of Contents
2
Grading Security Services and Mechanisms Symmetric Cipher Model Substitution techniques Transposition Techniques Stream and Block Ciphers
Security Attacks
3
Security Attacks
Snooping
Traffic Analysis
Modification
Masquerading
Replaying
Denial of Service
Confidentiality Integrity Availability
Security Services and Mechanisms
4
International Telecommunication Union Telecommunication Standardization (ITU-T) Provides:
• Services
• Mechanisms
Security Services
5
Authentication - assurance that the communicating entity is the one claimed
Access Control - prevention of the unauthorized use of a resource
Data Confidentiality –protection of data from unauthorized disclosure
Data Integrity - assurance that data received is as sent by an authorized entity
Non-Repudiation - protection against denial by one of the parties in a communication
Security Mechanisms
6
Specific security mechanisms:• Implemented on specific layer (OSI model)
• Encipherment, digital signatures, access controls, data integrity, authentication exchange, routing control, notarization
Pervasive security mechanisms:• Not related to a specific layer
• Trusted functionality, security labels, event detection
Model for Network Security
7
Model for Network Security
8
Using this model requires us to: • Design a suitable algorithm for the security
transformation.
• Generate the secret information (keys) used by the algorithm.
• Develop methods to distribute and share the secret information.
• Specify a protocol enabling the principals to use the transformation and secret information for a security service.
9
Symmetric Cipher Model
Symmetric Cipher Model
10
Known as:• Conventional Encryption
• Single-Key Encryption
Plaintext• Original text/msg
Ciphertext• Coded msg
Enciphering/Encryption• The process of converting the plaintext to ciphertext
Deciphering/Decryption • The process of converting the ciphertext to plaintext
Symmetric Cipher Model (Cont.)
11
Cryptography • The developed encryption schemes
Cryptanalysis • Techniques used to get the plaintext out of the ciphertext without
prior knowledge to the encryption scheme (breaking the code)
Cryptology • Both the cryptography and cryptanalysis
More Definitions
12
Unconditional Security • The ciphertext provides insufficient information to
uniquely determine the corresponding plaintext.
Computational Security • The time needed for calculations is greater than
age of universe
Symmetric Cipher Model (Cont.)
13
Symmetric Cipher Model
14
Requirements • Strong Key the opponent can not figure it out even if he/she has
a number of ciphertexts
• The key must be exchanged through a secure channel
• Y = E(K,X) ~ Y = EK(X)
• X =D(K,Y) ~ X = DK(Y)
Brute Force Search
15
Always possible to simply try every key Most basic attack, proportional to key size
16
Substitution Ciphers
Lets have Fun
17
You are spying on your friend Ahmed while he is chatting with John, you received the following message:
“Ygjcxgvqmnnvjgrgumfgpv”
Can you decrypt this message?
Answer
18
Ahmed is telling John:
“Ygjcxgvqmnnvjgrgumfgpv”
“We have to kill the president” Encryption Key:
• Replacement Table Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ Ciphertext CDEFGHIJKLMNOPQRSTUVWXYZAB
Encryption Technique • Each letter is replaced by the second one after it
• Remove blanks
Caesar Cipher
19
Earliest known substitution cipher by Julius Caesar first attested use in military affairs replaces each letter by 3rd one after it
E.g.meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
Caesar Cipher (Cont.)
20
Transformation :
Mathematically give each letter a numbera b c d e f g h i j k l m0 1 2 3 4 5 6 7 8 9 10 11 12n o p q r s t u v w x y Z13 14 15 16 17 18 19 20 21 22 23 24 25
Then have Caesar cipher as:C = E(p) = (p + k) mod (26)p = D(C) = (C – k) mod (26)
Caesar Cipher (Cont.)
21
Cryptanalysis
• Only have 26 possible ciphers
•A maps to A,B,..Z
• Could simply try each in turn
Monoalphabetic Cipher
22
Rather than just shifting the alphabet Could shuffle (jumble) the letters arbitrarily Each plaintext letter maps to a different random
ciphertext letter The key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZNPlaintext: ifwewishtoreplacelettersCiphertext: WIRFRWAJUHYFTSDVFSFUUFYA
Monoalphabetic Cipher Security
23
now have a total of 26! = 4 x 1026 keys with so many keys, might think is secure but would be !!!WRONG!!!
Language Characteristics Problem
• Using the occurrence frequency of each letter , we can deduce the letters in the ciphertext
English Letter Frequencies
24
Playfair Cipher
25
Invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair.
Encrypts multiple letters
Uses Playfair Matrix
Uses some of the rules to interpret the matrix
Playfair Key Matrix
26
A 5X5 matrix of letters based on a keyword Fill in letters of keyword (Avoid repetition) Fill rest of matrix with other letters E.g. using the keyword MONARCHY
M O N A R
C H Y B D
E F G I/J K
L P Q S T
U V W X Z
Playfair Rules
27
Plaintext encrypted two letters at a time: • if a pair is a repeated letter, insert a filler like 'X',
• eg. "balloon" encrypts as "ba lx lo on"
• If both letters fall in the same row, replace each with letter to right (wrapping back to start from end), • eg. “ar" encrypts as "RM"
• If both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom), • eg. “mu" encrypts to "CM"
• Otherwise each letter is replaced by the one in its row in the column of the other letter of the pair,• eg. “hs" encrypts to "BP", and “ea" to "IM" or "JM" (as desired)
Group Activity
28
Based on Playfair encryption, encrypt the word
“Hello”
Key :
Note: The key is an arrangement of all of the alphabetic letters
L G D B A
Q M H E C
U R N I/J F
X V S O K
Z Y W T P
Answer
29
Step 1: Group the letters
• He ll o
• 1st rule repeated letters ll
• He lx lo Step 2: find the corresponding text in the key
• He EC - rule 2 H and e on the same row (replace each with letter to right) EC
• Lx QZ -- rule 3 L and x at the same column (replace each with the letter below it) QZ
• loBX -- rule 4 l and o at different rows and columns (replaced by the one in its row in the column of the other letter of the pair)
E (Hello) “ECQZBX”
Security of the Playfair Cipher
30
Security much improved over monoalphabetic
Since have 26 x 26 = 676 diagrams
Was widely used for many years (eg. US & British military in WW1)
It can be broken, given a few hundred letters since still has much of plaintext structure
Polyalphabetic Ciphers
31
Another approach to improving security is to use multiple cipher alphabets
Makes cryptanalysis harder with more alphabets to guess and flatter frequency distribution
Use a key to select which alphabet is used for each letter of the message
Use each alphabet in turn Repeat from start after end of key is reached
Vigenère Cipher
32
Simplest polyalphabetic substitution cipher effectively multiple caesar ciphers key is multiple letters long K = k1 k2 ... kd ith letter specifies ith alphabet to use use each alphabet in turn repeat from start after d letters in message decryption simply works in reverse
33
Example
34
eg using repeated keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
From the previous table lookup the key letter then the
plain text letter.
The cipher letter is the intersection letter
Security of Vigenère Ciphers
35
have multiple ciphertext letters for each plaintext letter
Letter frequencies are obscured
But not totally lost
Autokey Cipher
36
Ideally want a key as long as the message Vigenère proposed the autokey cipher The keyword is prefixed to message as key Still have frequency characteristics to attack
Eg. given key deceptive
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLA
One-Time Pad
37
Select a random key that is equal to the message length.
Use a table structure such as Vigenère table
Problems: • Generating long random keys
• Bandwidth problem sending the key as long as the Msg
38
Transposition/Permutation Ciphers
Transposition (Cont.)
39
The letters of the message are rearranged
Columnar transpositionThe number of columns is required
Example:
THIS IS A MESSAGE TO SHOW HOW A COLMUNAR TRANSPOSITION WORKS
Transposition (Cont.)
40
T H I S I S A M E S S A G E T O S H O W H O W A C O L M U N A R T R A N S P O S I T I O N W O R K S
tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasna
Group Activity
41
Given the following message
“ This is the second lecture”
Divide the message onto a block of 5 letters block Transpose the message Use Autokey cipher to encrypt the result
• Key : “ NetworkSecurity”
Stream Vs. Block Ciphers
42
Stream converts one symbol of plaintext into a symbol of ciphertext
Block encrypts a group of plaintext symbols as one block.
Reading materials
43
Stallings Chapter 1
Chapter 2